Olefins, such as ethylene and propylene, have become major feedstocks in the organic chemical and petrochemical industries. Of these, ethylene is by far the more important chemical feedstock, since the requirements for ethylene feedstocks are about double those for propylene feedstocks. Consequently, feedstocks for the production of ethylene are in relatively short supply.
Numerous suggestions have been made for the production of ethylene from various feedstocks by a variety of processes.
At the present time, ethylene is produced almost exclusively by dehydrogenation or pyrolysis of ethane and propane, naptha and, in some instances, gas oils. About 75% of the ethylene is produced by steam cracking of ethane and propane derived from natural gas. However, natural gas contains as little as 5 volume percent and, in rare instances, as much as 60 volume percent of hydrocarbons other than methane, the majority of which is ethane. However, typical natural gases contain less than about 12 to 15% of ethane. In addition to the relatively small quantities of ethane and propane available for use, separation of these components from natural gas is itself an expensive and complex process, usually involving compression and expansion, cryogenic techniques and combinations thereof.
It would, therefore, be highly desirable to be able to produce ethylene from the much more abundant methane. However, methane's high molecular stability, compared to other aliphatics, makes its use in ethylene production difficult and no significant amount of ethylene is produced commercially from methane at the present time.
Pyrolytic or dehydrogenative conversion of methane or natural gas to higher hydrocarbons has been proposed. However, relatively severe conditions, particularly temperatures in excess of 1000.degree. C., are required. In addition, such reactions are highly endothermic and thus energy intensive. In order to reduce the severity of the conditions, particularly temperature, numerous proposals to catalyze pyrolytic reactions have been made. Some of these processes do, in fact, reduce the required temperatures, but the conversion of methane and the selectivity to ethylene are still quite low.
Another promising approach is the oxidative conversion of methane or natural gas to higher hydrocarbons. However, these techniques are still in the developmental stage and experimentation is hampered by differences of opinion and lack of a complete understanding of the process. For example, most workers in the art refer to the process as "oxidative coupling". However, there is little agreement with regard to the function performed by the oxygen and how this function is performed. Accordingly, the terminology, "oxidative coupling", will be avoided herein, and the present process, irrespective of the function of the oxygen or of the manner in which it performs its function, will be referred to as "oxidative conversion of methane". In such processes, it is conventional to contact the methane with solid materials. The nature of these contact materials, the function thereof and the manner in which such function is performed are also subject to diverse theories. For example, workers in the art refer to the function of the contact material as a purely physical phenomenon, in some cases as adsorption-desorption, either of atomic or molecular oxygen and either on the surface or occluded within the solid material, oxidation-reduction utilizing multivalent metals capable of oxidation-reduction, adsorption and desorption of the hydrocarbons on the solid materials, a free radical mechanism, etc. Consequently, the solid materials, utilized in the process, are referred to as "contact materials", "promoters", "activators" and "catalysts". Accordingly, in order to avoid functional categorization, the terms "solid contact material" or "solid contact materials" will be utilized in the present application.
Based on the prior art, oxidative conversion of methane results in the formation of a variety of products. The most readily produced products are carbon dioxide, carbon monoxide and/or water and methanol, formaldehyde and other oxygenated hydrocarbons in combination with one or more of carbon dioxide, carbon monoxide and water. Higher hydrocarbons, particularly ethylene and ethane, are either not formed or are formed in such small quantitites that commercially viable processes have not been developed to date. Along with poor selectivity to higher hydrocarbons, particularly ethylene and ethane and still more particularly to ethylene, such processes also result in low conversions of the methane feed.
It is clear from the above that the suitability of particular contact materials is unpredictable. In addition to being dependent upon the type of contact material, the conversion of methane and selectivity to particular products also depends upon the conditions and the manner in which the reaction is carried out, and there is also little basis for predicting what conditions or what mode of operation will result in high conversions and selectivity to particular products.